Method of neutralizing disturbing frequencies in communication circuits



Dec. 15, 1931. w p CANNON 1,836,575

METHOD OF NEUTRALIZING DISTURBING FREQUENCIES IN COMMUNICATION CIRCUITS Filed Jan. 31, 1930 2 Sheets-Sheet 1 lll gnuemfoc Dec. 15, 1931. w. D. CANNON 1,336,575

METHOD OF NEUTRALIZING DISTURBING FREQUENCIES IN COMMUNICATION CIRCUITS Filed Jan. 31, 1930 2 Sheets-Sheet 2 atloz "4,1

Patented Dec. 15, 1931 UNITED STATES PATENT OFFICE WILLIAM D. CANNON, OF METUCHEN, NEW JERSEY, ASSIGNOR TO THE WESTERN UNION TELEGRAPH COMPANY, OF NEW YORK, N. Y., A. CORPORATION OF NEW YORK METHOD OF N EUTRALIZING DISTURBING FREQUENCIES IN COMMUNICATION CIRCUITS Application filed January 31, 1980. Serial No. 425,044

My invention relates to an improved methd of and means for protecting communication circuits from interfering currents which ma y be induced in them by paralleling power circuits or from other sources. The method is particularly adapted to the neutralization of a plurality of frequencies.

More specifically, my invention relates to a multiple frequency neutralizing transformer; it comprises a neutralizing transformer in combination with a shunting network designed to make the transformer function effectively at a plurality of. frequencies.

One of the objects of the invention is to neutralize in a communication circuit, or group of circuits, disturbing currents induced from a plurality of sources.

Another object is to neutralize in such communication circuits a fundamental power frequency and its harmonics.

It is a further object to accomplish effective neutralization of a plurality of disturbing frequencies while keeping disturbances occasioned by crossfire between the communication circuits at a minimum.

My invention also comprises a convenient method of applying a shunting network to a neutralizing transformer of the type disclosed, so as to provide for the effective neutralization of disturbing frequencies commonly encountered in communication circuits.

35 My invention is illustrated in the accompanying drawings in which Fig. 1 represents diagrammatically the neutralizing transformer and its shunting network applied to a plurality of disturbed lines which are subject to a plurality of disturbing frequencies.

Fig. 2 is a further diagrammatic representation of the neutralizing transformer and its shunting network, which will be used to ex- 45 plain the method of determining mathematically the electrical values of the elements of the shunting network.

Fig. 3 is a diagrammatic representation of the transformer primary and one of the branches of the shunting network, intended to represent its electrical make up when account is taken of the copper and iron losses.

Fig. 4 is a similar representation of a transformer designed to provide neutralization of two frequencies.

Fig. 5 is a graphical representation of the electrical characteristics of the various branches of the shunting network, which will be used in explaining a convenient method of applying a shunting network to the primary of a neutralizing transformer.

It has long been the practice to neutralize disturbing currents of a single frequency in a communication circuit by means of neutralizing transformers. An improved transformer of this type is described in Patent No. 1,714, 17 6 to J. W. Milnor, and is again illustrated in its essentials in the Fig. 1 accompanying this description.

Referring to Fig. 1, the distance AB constitutes an exposure of a group of communication circuits to a disturbing power line. '1 represents the neutralizing transformer containing a secondary coil S for each conductor of the communication circuits to be protected, and a primary coil, or coils, P which is in series with a primary wire. This primary wire, of very low resistance, and grounded at each end of the exposure, serves to pick up a disturbing voltage similar to that induced in the signal conductors and applies it to these conductors in opposing direction by means of the primary coil, thereby neutralizing the voltages induced in the other wires. A condenser C is usually placed in parallel with the primary coil and adjusted to such a value that the neutralizing voltage is exactly 180 out of phase with the disturbing voltage. It follows then that the circuit comprising the coil P and the condenser 0 will be antiresonant at the ower frequency, and will thus have at that frequency a resistive impedance of maximum value. A fortunate result of this high impedance is that at this frequency a maximum proportion of the voltage present in the primary wire is applied across the transformer primary and is thus made effective for neutralizing purposes.

A neutralizing transformer functions at any frequency for which its primary has a high impedance, as compared with. the impedance of the primary wire, and zero reactance. The transformer just described is effective at only one frequency.

Communication circuits are frequently-subjected to disturbances of more than one frequency. They may be exposed to power systems using different frequencies, or if there are harmonics of the power frequencies pres,- ent in considerable strength, these may also be induced in the communication circuits.

In the present invention, the transformer is made effective at a number of different frequencies. This is accomplished by connecting a network in parallel with the primary coil, which when considered in conjunction with the inductance of the primary has a high impedance and substantially zero reactance at each disturbing frequency.

As illustrated in Fig. 1, this network is made up of a first branch comprising a condenser C a second branch comprising a series arrangement of inductance L and condenser C and a third branch comprising inductance L and condenser 0 If the transformer is to function at a large number of disturbing frequencies, additional branches will be connected in parallel to the branches illustrated and to the primary P, as indicated diagrammatically in Fig. 1 by the extended terminals in full lines and the dotted line extending vertically downward therebetween.

Fig. 2 shows a primary coil L of a neutralizing transformer shunted by a network comprising a number of branches. If the network contained only branch A, comprising the condenser C alone, the primary circuit would have high impedance at but one frequency, and the neutralizing transformer would be effective at that frequency only. It is possible however to devise a network containing one branch for each frequency that is to be neutralized, which when connected in parallel with the transformer primary will possess a high impedance at each of the prescribed frequencies. The network of Fig. 2 containing branches A to N has been found to fulfill this condition. It will now be shown that this network containing n branches can be made to have a high impedance .and substantially zero reactance at n frequencies.

Under operating conditions practically no load is placed on the transformer secondaries, and since the primary coil resistance is small, it may be represented for simplicity as an inductance L. It is convenient to represent the condition of high impedance and zero resistance across the transformer primary by the condition of zero susceptance. The susceptance of branch A taken alone in conjunction with L is:

j='./- 1 and 0 21 times the frequency to be neutralized.

The susceptance of any element K of the remaining elements is The susceptance of the entire network is the sum of the component susceptances; if this sum is set equal to zero and the resulting equation divided by 7', then in which .'1 'L 0 0) 1-L,,U,,w Clearing of fractions.

ing transformer is shunted by a network,

containing or branches, which meets the above condition, neutralization will be effective at n frequencies.

In practice a small amount of resistance will always be present in the transformer primary and in the inductance coils. This resistance however will not affect the validity of the method, and when it is within limits easily realizable commercially, will have but negligible effect on the value of the elements for the given n frequencies to be neutralized. That this is true may be shown by considering Fig. 3 which contains the reactance elements L and C of Fig. 2 but has a resistance R (simulating both the copper and iron losses at the frequency considered) associated with the inductance. When the resistance is considered the circuit will have a finite impedance which must be resistive if the voltage in the secondary circuit or circuits is to be 180 degrees out of phase with the voltage of the primary or control circuit. If the impedance is to be made resistive the imaginary part of the admittance equation which is,

53+ 0 =0 (6) For given values of L C and R this may be accomplished at one frequency onl i. e.,

onl one frequency may be effective y neutra ized. Equation (6) may therefore be where (0 is the frequency to be neutralized. Since it is practicable commercially to make 1 1 1 small at commercial frequencies, the square of A L w may be neglected and (n approximates the value of {L 0 which it should have if R is zero. The real part of (5) then becomes which can be made sufiiciently high for satisfactory compensation;

Applying the same theory to a network like that illustrated in Fig. 4, which is designed to provide neutralization at two fre-' quencies, the admittance is found to be;

1 1 Ya) J01l R J11. L L120? 20, 020) R12 R22 1 1 i 2 1 1 L w 7 0 m L19" 2 02w 2 R R When the susceptance is zero-- 1 1 L w 0 w- 0 11 2+ R 2 l 2 (ml 3;)

- When the values of the elements are fixed Y (8) in accordance with this equation, there will 1 be two frequencies at which this equation will (91L! hold. These frequencies are approximately 3 L1+L2+ 0 +\/(L1+L2 4OI-IIL2 w 2 2 2 2 (12) 2 2O L L and I 1+ z+ b /(L,+L gi 2: 2 z

3 2C L L that is As before, 1

20 11W it 2 21 R, (w11; l a Smce R in an efiicient design should be small, and so wl l R1 1 is small, the impedance of Z, approximates 1 1 may be neglected. R L and C form a B series resonant circuit which may have a low -admittance at the frequencies :0 and m at which compensation is sought; i.

1. ar will be small and its square may be neglected. The relation among circuit constants and frequency of equation (11) with these three approximations then becomes which is identical with that obtained from the general equation previously deduced when the resistances are assumed as zero.

Using the same approximations referred to above, the admittance of the system at frequencies a); and w, becomes Since both terms of each of these equations are small, the impedance the circuit including R L and C particularl if this resonant frequency can be made to all at the point where crossfire is most severe.

It follows then that in a properly designed network such as shown in Fig. 4, the resistance will have but slight effect on the value of L L C and C to be chosen for neutralizing two frequencies, the impedance will be sufiiciently high for neutralizing each of the two frequencies, and the reactance will be substantially zero.

By a continuation of the theory just detailed, it is possible to prove that a network such as Fig. 2, containing n branches will have a high impedance at 11. different freuencies and when used in conjunction with t e primary coil of a neutralizing transformer will be effective in neutralizing disturbing currents of n different frequencles.

The values of the inductances and capacities comprising the network to be used with e initial branch A of the network. First,

the'light lines, A, A, represent the admittance of the coil L and the condenser 0 while their sum is indicated by the dotted line A. A zero admittance is obtained at the point where the dotted line crosses the f uency axis and as a consequence the most e ective neutralization will occur at substantially this one frequency. If now the branch B is added to the network, the admittance of the added branch will be represented by the medium heavy lines B, B while the total admittance follows the two medium heavy dotted lines A+B. There are now two frequencies at which zero admittance occurs.

It will be noted that the original frequency calculated for the network A has been displaced somewhat to the left, that is, the first frequency for which neutralization was indicated has been reduced to a slightly lower value. When the third branch C is added to the network and the admittance curves 0, C shown in the very heavy lines are added to the two medium hea dotted lines A+B, three points of zero a ittance will result where the three heavy dotted lines cross the frequency axis, and the two oints reviously found will be shifted to a s ightly lower frequency.

It is apparent that as each additional branch is placed in the network, neutralization will be accomplished for an additional frequency and that the frequencies at which the previously existing branches accomp lished neutralization will be slightly altered.

his alteration however can be corrected as successive networks are added by sli ht readjustment of the capacity values so t at the frequencies at which zero admittance occurs can be made to agree with the disturbing frequencies.

As illustrative of practicable values of inductance and capacitance in combination to produce satisfactory compensation at a number of frequencies, the following numerical examples are given:

Disturbing power frequencies usually have known constant values such as the fundamental from one or more sources, the fundamental and certain of its harmonics or certain of the harmonics alone. It is referable to design a network. so that the isturbing voltages of certain fixed frequencies which are known to exist will be effectively suppressed while leaving the primary impedance low for all other frequencies, instead of designing the system so that a band or all frequencies are suppressed. This is a condition which is conducive to the reduct on of crossfire between the communication circuits inasmuch as the severity of the cross: fire is a direct function of the resistance of the primary system. At the points of hlgh impedance the primary system presents to crossfire currents the impedance of the primary wire and its ground return circuit, which in any case is kept small and thus crossfire should not be a serious factor.

From the foregoing it is apparent that when the network described herein is applied to the primary of a neutralizing transformer any of the disturbing frequencies commonly encountered in communication c1rcuits can be effectively neutralized. At the same time the coupling between the various secondary conductors which is inherent in the transformer will have a minimum effect in transferring energy between the various conductors.

Instead of connectingthe entire network in shunt to a single one of the transformer windings, the various elements may be distributed among several windings without regard to their function as primaries or secondaries. However if the turn ratio between primary and secondary windings differs from unity this fact must be taken into consideration in computing the values of the branches.

I claim:

1. In a system for neutralizing disturbing voltages, a disturbed circuit, a neutralizing circuit, said circuits being exposed to the same disturbing magnetic field. a transformer coupling said disturbed and neutralizing circuits, and a frequency selective network in shunt to the neutralizing circuit winding comprising a plurality of branches having impedance values such that the combination of transformer and network offers maximum impedances at a plurality of separated frequencies.

2. In a system for neutralizing disturbing voltages, a disturbed circuit, a neutralizing circuit, said circuits being exposed to the same disturbing magnetic field, a transformer coupling said disturbing and neutralizing circuits having its primary in the neutralizing circuit, and a network in shunt to said primary comprising 91 branches having constants such that the transformer has high impedance and substantially zero reactance at 4. In a system for neutralizing disturbing voltages, a disturbed circuit, a neutralizing circuit, said circuits being exposed to the same disturbing magnetic field, a transformer coupling said disturbed and neutralizing circuits having its primary in the neutralizing circuit, and a network in shunt to said primary comprising n branches having constants such that the transformer has high impedance and substantially zero reactance at n frequencies, and low impedance at frequencies intermediate said n frequencies.

5. In a system for neutralizing disturbing voltages of a plurality of frequencies, a disturbed conductor, a neutralizing conductor, and means for applying to the disturbed conductor at a plurality of non-adjacent frequencies substantially all of the Voltages of said frequencies present in the neutralizing conductor, said means including a transformer having a primary in said neutralizing conductor and a secondary in said disturbed conductor, and means for reducing the voltages induced between the secondary conductors at frequencies intermediate said nonadj acent' frequencies.

6. In a system for neutralizing disturbing voltages of a plurality of frequencies, a disturbed conductor, a neutralizing conductor and means for applying to the disturbed conductor at each of a plurality of frequencies a major portion of the voltages of said frequencies present in the neutralizing conductor at each of said plurality of frequencies, said means including a transformer having a primary in said neutralizing conductor, a secondary in said disturbed conductor, and a network in shunt to said primary comprising a capacity in one branch and an inductance and capacity in series in another branch.

7. In a system for neutralizing disturbing voltages of a plurality of frequencies, a disturbed conductor, a neutralizing conductor and means for applying to the disturbed conductor at each of a plurality of frequencies a major portion of the voltages of said frequencies present in the neutralizing conductor, said means including a transformer having a primary in said neutralizing conductor, a secondary in said disturbed conductor and a plurality of shunts across said primary one of which comprises a capacity and the others inductance and capacity in series.

8. li. a system for neutralizing disturbing voltages, a disturbing circuit which is the source of disturbing voltages of a plurality of frequencies, a communication circuit and a neutralizing circuit, a neutralizing transformer coupling said communication and neutralizing circuits and means associated with the windings of said transformer for producing in said communication circuit neutralizing voltages of said plurality of frequencies and for limiting to negligible values voltages of intermediate frequencies induced by one communication circuit into another communication circuit.

9. In a system for neutralizing disturbing voltages, a disturbing circuit which is the source of disturbing voltages of fundamental and harmonic frequencies, a communication circuit, a neutralizing circuit, a neutralizlng transformer coupling said communication and neutralizing circuits, and means associated with the windings of said transformer for producing in said communication circuit neutralizing voltages of said fundamental and harmonic frequencies and for limiting to negligible values voltages of intermediate frequencies induced by one communication circuit into another communication circuit.

10. In a system for neutralizing disturbing voltages, av disturbing circuit which is the source of disturbing voltages of a plurality of frequencies, a plurality of communication circuits, a neutralizing circuit, a neutralizing transformer comprising a primary in said neutralizing circuit and secondaries in each of said communication circuits, means including said neutralizing transformer and a frequency selective network for introducing into each of said communication circuits neutralizing voltages of said plurality of frequencies, and means comprising said frequency selective network for reducing crossfire between said communication circuits to a minimum.

11. In combination, a plurality of communication circuits located in a disturbing magnetic field, an auxiliary conductor adjacent to the communication circuits, and a transformer coupling said auxiliary conductor to the communication circuits, said aux iliary conductor having an abnormally low resistance as compared with the communication circuits whereby the creation of crossfire among the communication circuits is substantially avoided, and means associated with the auxiliary conductor winding of the transformer for producing a high impedance therein at each of a plurality of frequencies.

12. In combination, a. plurality of communication conductors subject to electrical disturbances, an auxiliary conductor adjaa plurality of disturbing frequencies and a low impedance at intermediate frequencies.

13. The method of neutralizing disturbances in communication circuits subjected to crossfire and to disturbing magnetic fields of a plurality of power frequencies, which comprises selectively inducing in said communication circuits neutralizing voltages of maximum values at said plurality of disturbing fre uencies while limiting the magnitudes of currents induced between the secondary conductors at frequencies intermediate said disturbing frequencies at frequencies intermediate said disturbing frequencies.

14. The method of applying a selective network to the primary of a neutralizing transformer to adapt the transformer for neutralizing operation at a plurality of disturbing frequencies which comprises adjusting the impedance of the transformer primary to a high value at a given disturbing frequency, by the addition of shunt ca acity, adjusting the impedance of the trans ormer anew to a. high value at a second disturbing frequency by the addition of inductance and capacity in series, thereby altering the impedance of the transformer for said first frequency, and readjusting the value of said shunt capacity to restore the high impedance at said first frequency.

15. The method of appl 'ng a selective network to the primary 0 a neutralizing transformer to adapt the transformer for neutralizing operation at a plurality of disturbing frequencies which comprises securing a high impedance in the transformer primary, at one frequency by the addition of shunt. capacity, and successively securing a high impedance at other disturbing frequencies by adding elements to the network thereby altering the impedance of the transformer for the preceding frequencies in the series, and compensating for said impedance changes between successive adjustments.

16. In a system for neutralizing disturbing voltages of a plurality of non-adjacent requencies, a disturbed circuit, a. neutralizing circuit, said circuits being exposed to the same disturbing magnetic field, transformers coupling said disturbed and neutralizing circuits, and networks comprising a plurality of frequency selective branches in shunt to the windings of said transformers,

the frequency selective branches of said networks having values such that the system ofi'ers maximum impedances at said non-adjacent frequencies and low impedances at all other frequencies.

In testimony whereof I afiix my signature.

WILLIAM D. CANNON. 

